A bizarre consequence of the appearance
of oxygen was the advent of the world's first nuclear reactors.
Nuclear power from its inception has rarely been described publicly
except in hyper-bole.

The impression has been given that to design
and construct a nuclear reactor is a feat unique to physical science
and engineering creativity. It is chastening to find that, in the Proterozoic, an unassertive community of modest bacteria built a set
of nuclear reactors that ran for millions of years.

This extraordinary event occurred 1.8 eons ago at a place now called
Oklo in Gabon, Africa, and was discovered quite by accident. At
Oklo,
there is a mine that supplies uranium mainly for the French nuclear
industry.

During the 1970s, a shipment of uranium from Oklo was
found to be depleted in the fissionable isotope 235U.

Natural
uranium is always of the same isotopic composition:

99.27 percent
238U

0.72 percent of 235U

traces of 234U

Only the 235U isotope can take part in the chain reactions necessary
for power production or for explosions. Naturally, the fissionable
isotope is guarded carefully and its proportion in uranium subjected
to thorough and repeated scrutiny.

Imagine the shock that must have
passed through the French atomic energy agency when it was
discovered that the shipment of uranium had a much smaller
proportion of 235U than normal.

Had some clandestine group in Africa or France found a way to
extract the potent fissionable isotope, and were they now storing
this for use in terrorist nuclear weapons?

Had someone stolen the
uranium ore from the mine and substituted spent uranium from a
nuclear industry elsewhere?

Whatever had happened, a sinister
explanation seemed likely.

The truth, when it came, was not only a
fascinating piece of science but must also have been an immense
relief to minds troubled with images of tons of undiluted 235U in
the hands of fanatics.

The chemistry of the element uranium is such that it is insoluble in
water under oxygen-free conditions, but readily soluble in water in
the presence of oxygen. When enough oxygen appeared in the
Proterozoic to render the ground water oxidizing, uranium in the
rocks began to dissolve and, as the uranyl ion, became one of the
many elements present in trace quantities in flowing streams.

The strength of the uranium solution would have been at most no more
than a few parts per million, and uranium would have been but one of
many ions in solution.

In the place that is now Oklo such a stream
flowed into an algal mat that included microorganisms with a strange
capacity to collect and concentrate uranium specifically.

They
performed their unconscious task so well that eventually enough
uranium oxide was deposited in the pure state for a nuclear reaction
to start.
When more than a "critical mass" of uranium containing the
fissionable isotope is gathered together in one place there is a
self-sustaining chain reaction.

The fission of uranium atoms sets
free neutrons that cause the fission of more uranium atoms and more
neutrons and so on.

Provided that the number of neutrons produced
balances those that escape, or are absorbed by other atoms, the
reactor continues.

This kind of reactor is not explosive; indeed it
is self-regulating. The presence of water, through its ability to
slow and reflect neutrons, is an essential feature of the reactor.
When the power output increases, water boils away and the nuclear
reaction slows down.

A nuclear fission reaction is a perverse kind of fire; it burns
better when well watered. The Oklo reactors ran gently at the
kilowatt-power level for millions of years and used up a fair
amount of the natural 235U in doing so.

The presence of the Oklo reactors confirms an oxidizing environment.
In the absence of oxygen, uranium is not water soluble. It
is just as well that it is not; when life started 3.6 eons back,
uranium was much more enriched in the fissile isotope 235U.

This
isotope decays more rapidly than the common isotope 238U, and at beginning the proportion of fissile uranium was not 0.7
percent as now but 33 percent. Uranium so enriched could have been
the source of spectacular nuclear fireworks had any bacteria then
been unwise enough to concentrate it. This also suggests that the
atmosphere was not oxidizing in the early Archean.

Bacteria could not have debated the costs and benefits of nuclear
power. The fact that the reactors ran so long and that there was
more than one of them suggests that replenishment must have occurred
and that the radiation and nuclear waste from the reactor was not a
deterrent to that ancient bacterial ecosystem.

(The distribution of
stable fission products around the reactor site is also valuable
evidence to suggest that the problems of nuclear waste disposal now
are nowhere near so difficult or dangerous as the feverish
pronouncements of the antinuclear movement would suggest.)

The Oklo
reactors are a splendid example of geophysiological homeostasis.